This fixes issues identified in the predefined migration for
postgres v2-v3 including the following:
### relation already exists
Can error with the following:
```ts
{
err: [DatabaseError],
msg: 'Error running migration 20250502_020052_relationships_v2_v3 column "relation_id" of relation "table_name" already exists.'
}
```
This was happening when you run a migration with both a required
relationship or upload field and no schema specified in the db adapter.
When both of these are true the function that replaces `ADD COLUMN` and
`ALTER COLUMN` in order to add `NOT NULL` constraints for requried
fields, wasn't working. This resulted in the `ADD COLUMN` statement from
being being called multiple times instead of altering it after data had
been copied over.
### camelCase column change
Enum columns from using `select` or `radio` have changed from camelCase
to snake case in v3. This change was not accounted for in the
relationship migration and needed to be accounted for.
### DROP CONSTRAINT
It was pointed out by
[here](https://github.com/payloadcms/payload/issues/10162#issuecomment-2610018940)
that the `DROP CONSTRAINT` needs to include `IF EXISTS` so that it can
continue if the contraint was already removed in a previous statement.
fixes https://github.com/payloadcms/payload/issues/10162
3edfd7cc6d